The present invention relates to a multilayer filter for use in a high frequency circuit of a mobile communication apparatus such as a portable telephone.
When connecting two or more filters, each having different band pass region, to a conventional multilayer filter, a phase shifter has been provided as an external device at the respective input/output ports in order not to affect each other""s band pass region.
Further, as shown in FIG. 20, two band pass filters 61, 62 have been employed for matching the impedance so as the two band pass regions, viz. a low band pass region 31 and a high band pass region 32 of FIG. 19, do not give influence to each other.
However, if each of the input/output terminals of the respective filters is connected with an external phase shifter, the overall size of an entire filter becomes large, rendering it unsuitable for use in a mobile communication apparatus where the small-size, light-weight and thin-shape are the essential requirements.
In a configuration where two band pass filters 61, 62 are provided as shown in FIG. 20, the designing consideration is focussed only on the impedance matching between the low band pass region 31 and the high band pass region 32. Therefore, the amount of attenuation remains insufficient with respect to a band region 33 locating between the low band pass region 31 and the high band pass region 32. Thus it deteriorated the characteristics of high frequency circuit in a mobile communication apparatus.
The present invention addresses the above described drawbacks, and offers a small multilayer filter with which the amount of attenuation is sufficient in a region other than band pass region, while the insertion loss characteristic caused as a result of insertion of two or more band pass regions is not deteriorated.
The invented multilayer filter comprises a plurality of strip lines provided on a dielectric layer, a side electrode connected with an end of input pattern and output pattern which patterns are coupled with an open end of the strip line via dielectric layer, and an electrode pattern connecting said side electrode with input electrode and output electrode. With the above described structure, a phase shifter of a filter may be constituted within the filter, making the filter small in size.
In the invented multilayer filter, an attenuation peak is placed in a region other than the band pass region. Therefore, a sufficient amount of attenuation is ensured outside the band pass region without deteriorating the insertion loss characteristic of the band pass region.